I. Field of the Invention
The present invention relates to data communication. More particularly, the present invention relates to a novel and improved method and apparatus for reverse link rate scheduling in a communication system having a variable data transmission rate.
II. Description of the Related Art
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the "TIA/EIA/IS-95A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", hereinafter referred to as the IS-95A standard. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS", and U.S. Pat. No. 5,103,459, entitled "SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM", both assigned to the assignee of the present invention and incorporated by reference herein.
The IS-95A standard is designed to optimize voice communication and many important system design parameters are selected to achieve that goal. For example, since time delay between speakers cannot be tolerated, processing delays are sought to be minimized. Each user is assigned a transmission rate capable of carrying speech data for the duration of the call. Upon termination of the call, the assigned transmission rate can be reassigned to another user.
In the CDMA system, users communicate with one another through remote stations which, in turn, communicate with each other through one or more base stations. In this specification, base station refers to the hardware with which the remote stations communicate. Cell refers to the hardware or the geographic coverage area, depending on the context in which the term is used.
In the CDMA system, communications between users are conducted through one or more cells which are serviced by base stations. A first user on one remote station communicates to a second user on a second remote station, or a standard telephone, by transmitting voice data on the reverse link to a cell. The cell receives the voice data and can route the data to another cell or a public switched telephone network (PSTN). If the second user is on a remote station, the data is transmitted on the forward link of the same cell, or a second cell, to the second remote station. Otherwise, the data is routed through the PSTN to the second user on the standard phone system. In IS-95A systems, the forward link and the reverse link are allocated separate frequency and are independent of one another.
The remote station communicates with at least one cell during a communication. CDMA remote stations are capable of communicating with multiple cells simultaneously during soft handoff. Soft handoff is the process of establishing a link with a new cell before breaking the link with the previous cell. Soft handoff minimizes the probability of dropped calls. The method and system for providing a communication with a remote station through more than one cell during the soft handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled "MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM," assigned to the assignee of the present invention and incorporated by reference herein. Soft handoff impacts various aspects of the CDMA system design because considerations must be given to the status and capacity of each of the multiple cells involved in the soft handoff when a new allocation of resource is made.
In accordance with the IS-95A standard, each remote station is assigned a transmission rate of 28.8 Kbps on the reverse link for the duration of the communication with a cell. Using a rate 1/3 convolutional encoder, the data rate of each remote station approaches 9.6 Kbps. Although not specified by the IS-95A standard, higher data rates can by supported by the use of other code rates. For example, a data rate of 14.4 Kbps is achieved by using a rate 1/2 convolutional encoder.
The CDMA system is a spread spectrum communication system. The benefits of spread spectrum communication are well known in the art and can be appreciated by reference to the above cited references. The CDMA system must work within the pre-existing non-contiguous frequency allocation in the cellular band. By design, a CDMA system which conforms to IS-95A standard is allotted a 1.2288 MHz bandwidth to fully utilize the cellular band. The reverse link refers to transmission from the remote stations to a cell. On the reverse link, the 28.8 Kbps transmission rate is spread over the entire 1.2288 MHz system bandwidth.
On the reverse link, each transmitting remote station acts as an interference to other remote stations in the network. Therefore, the reverse link capacity is limited by the total interference which a remote station experiences from other remote stations. The IS-95A CDMA system increases the reverse link capacity by transmitting fewer bits, thereby using less power and reducing interference, when the user is not speaking.
To minimize interference and maximize the reverse link capacity, the transmit power of each remote station is controlled by two power control loops. The first power control loop adjusts the transmit power of the remote station such that the signal quality, as measured by the energy-per-bit-to-noise-plus-interference ratio, E.sub.b /(N.sub.o +I.sub.o), of the signal received at the cell is maintained at a constant level. This level is referred to as the E.sub.b /(N.sub.o +I.sub.o) set point. The second power control loop adjusts the set point such that the desired level of performance, as measured by the frame-error-rate (FER), is maintained. The power control mechanism for the reverse link is disclosed in detail in U.S. Pat. No. 5,056,109, entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM", assigned to the assignee of the present invention and incorporated by reference herein.
The user on each remote station transmits at a different bit rate depending on the level of speech activity in the conversation of that user. A variable rate speech vocoder provides speech data at full rate when the user is actively speaking and at low rate during period of silence, e.g. pauses. The variable rate vocoder is described in detail in U.S. Pat. No. 5,414,796, entitled "VARIABLE RATE VOCODER," assigned to the assignee of the present invention and incorporated by reference herein.
For the CDMA system, the reverse link capacity for voice communication between the remote stations and the cell, as measured by the number of users supportable by the cell, can be determined by the transmission rate of the user on each remote station. This is because other parameters determinative of the reverse link capacity are fixed by the system design or are given. For example, the maximum transmit power available for each remote station is limited by FCC regulations and also by system design constraints. The E.sub.b /(N.sub.o +I.sub.o) required to maintain the desired level of performance is dependent on the channel condition which cannot be controlled. Finally, the CDMA system bandwidth of 1.2288 MHz is selected by design.
The amount of speech activity at any given moment is non-deterministic. Also, there is typically no correlation in the level of speech activities among users. Therefore, the total power received at the cell from all transmitting remote stations varies over time and can be approximated as a Gaussian distribution. During periods of active speech, the remote station transmits at higher power and causes more interference to other remote stations. More interference lowers the received E.sub.b /(N.sub.o +I.sub.o) of other remote stations, which increases the probability of frame errors in the voice data received by the cell if the power control is not able to track the dynamics. Therefore, the number of users able to have access to the communication system is limited so that only a small portion of the transmitted frames is lost through excessive interference.
Limiting the reverse link capacity to maintain the desired frame error rate (FER) has the effect of forcing the cell to operate at less than full capacity, on the average, thereby under-utilizing the reverse link capacity. In the worse case, up to half of the reverse link capacity is wasted to maintain a headroom of up to 3 dB. The headroom is the difference between the maximum power the cell can receive and the average power the cell actually receives. The headroom is only utilized during the period when the speech activities of the users at the remote stations are high.
Data communication within the CDMA system has different characteristics than voice communication. For example, data communication is typically characterized by long period of inactivity, or low activity, punctuated by high bursts of data traffic. An important system requirement for data communication is the transmission delay required to transfer the burst of data. Transmission delay does not have the same impact in data communication as it does for voice communication, but it is an important metric for measuring of the quality of the data communication system.
A method for transmitting data traffic in code channel frames of fixed size, wherein the data source provides data at a variable rate, is described in detail in U.S. Pat. No. 5,504,773, entitled "METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION", assigned to the assignee of the present invention and incorporated by reference herein. Data is partitioned into data frames and each data frame may be further partitioned into data portions. The data portions are then encoded into code channel frames which can be 20 msec wide. At the 28.8 Kbps symbol rate, each 20 msec wide code channel frame contains 576 symbols. A rate 1/2 or rate 1/3 convolutional encoder is used to encode the data, depending on the application. Using a rate 1/3 encoder, the data rate is approximately 9.6 Kbps. At the 9.6 Kbps data rate, there are 172 data bits, 12 cyclic redundancy check (CRC) bits and 8 code tail bits per code channel frame.
High speed data transmission on the reverse link can be achieved by concurrently transmitting the data traffic over multiple code channels. The use of multiple code channels for data transmission is disclosed in U.S. patent application Ser. No. 08/656,649, entitled "METHOD AND APPARATUS FOR PROVIDING RATE SCHEDULED DATA IN A SPREAD SPECTRUM COMMUNICATION SYSTEM", filed May 31, 1996, and in U.S. patent application Ser. No. 081654,443, entitled "HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM", filed May 28, 1996, both assigned to the assignee of the present invention and incorporated by reference herein.
The demand for the reverse link continuously changes over time, due to variations in the level of voice activities. The inefficient use of the reverse link can be improved by transmitting data traffic during the period of low voice activity. To avoid degradation in the quality of the voice communication, the data transmission should be dynamically adjusted to match the available reverse link capacity of the cell.
In dealing with large sporadic bursts of data traffic, a system should be designed with the capability to transmit at high data rates and the ability to allocate the reverse link capacity to the users whenever requested, based on the availability of the capacity. In a CDMA system, the design should address other existing system considerations. First, since voice communication cannot tolerate extensive delay, priority should be given to transmission of voice data over transmission of any data traffic. Second, since the amount of voice activity at any given moment is unpredictable, the reverse link should be continuously monitored and the data transmission should be dynamically adjusted so that the reverse link capacity is not exceeded. Third, since the remote station may be in soft handoff between multiple cells, the data transmission rate should be assigned based on the reverse link capacity of each of the base stations participating in the soft handoff. These and other considerations are addressed by the present invention.